1. Technical Field
The present invention relates to a cellulose acylate film having optical anisotropy and capable of being directly stuck to a polarizing film, and a method for producing thereof, and to a retardation film, a polarizer, and a liquid crystal device using the cellulose acylate film.
2. Background Art
A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films.
Of those, cellulose ester films having suitable moisture permeability can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers.
On the other hand, when cellulose acylate film is applied to optical use, for example, in retardation films, supports for retardation films, protective films for polarizers and liquid crystal display devices, the control of their optical anisotropy is an extremely important element in determining the performance (e.g., visibility) of display devices. With the recent demand for broadening the viewing angle of liquid crystal display devices, improvement of retardation compensation in the devices is desired, for which it is desired to suitably control the in-plane retardation Re (this may be simply referred to as Re) and the thickness-direction retardation Rth (this may be simply referred to as Rth) of the retardation film to be disposed between a polarizing film and a liquid crystal cell. In addition, it is desired to control properly not only optical characteristics of the film but also physical characteristics of the film.
As a method of manufacturing the film having these optical properties, for example, methods of extending the film in a longitudinal direction or a transverse direction (see JP-A-2002-127244 and JP-A-2004-243628), sequentially extending the film in two axes (see JP-A-2005-330411), simultaneously extending the film in two axes (see JP-A-2005-22037), and extending the film in a thickness direction thereof (see JP-A-5-157911 and JP-A-2000-231016) are disclosed. However, in the film manufactured by these methods, there are problems that a balance control between Re and Rth is not enough and both of the optical property and the dynamic property of matter of the film are not improved.